Oxygen-containing or combustible gases are blown into a rotary kiln through its shell or mantle by means of shell pipes (shell tubes) or nozzle blocks at various locations which are distributed over the length of the rotary kiln.
Shell pipes extend radially through the shell of the kiln and have outlets disposed approximately at the center of the kiln (or inwardly of the kiln wall) so that these outlets always lie in the free kiln space.
The shell pipes may consist of simple pipes which are used to inject oxygen-containing gases, generally air, or may comprise shell burners for injecting combustible gases.
Nozzle blocks also extend radially through the shell of the kiln but their outlet openings are substantially flush with the inner surfaces of the lining of the kiln so that they may be temporarily covered by the charge of the kiln.
In most cases, several nozzle blocks form an annular series. Air is usually supplied by blowers, which are mounted on and revolve with the wall of the kiln. Combustible gases must be supplied through wiping seals at the ends of the kiln.
In numerous processes carried out in rotary kilns, particularly the direct reduction of iron oxides at temperatures below the softening point of the charge to produce sponge iron, the temperature profile in the kiln must be exactly controlled; this requires an exact control of the rates at which gases are injected at various locations.
It is known to measure the temperatures at various locations spaced along the rotary kiln by thermocouples and to supply the signal currents thus obtained to a control station via a closed (endless) slip ring, a segmented slip ring and stationary taps.
Reference may be had to U.S. Pat. No. 4,118,986, the art cited therein and the references of its class and subclass. Each thermocouple can be connected at one terminal to one of the segments.
The kiln is provided with shell pipes, which are spaced along the kiln and associated with the respective thermocouples. Each shell pipe is supplied with air from a shell blower.
The rate at which air is supplied to each shell pipe is controlled by a throttle valve which is connected to the control station by similar slip rings and receives from the control station a control command in dependence on the measured temperature (German Patent Publication No. 23 57 834).
Opened German application No. 23 34 676 discloses that the throttle valves can be adjusted by three-phase alternating current delivered via three segmented slip rings. The position of the throttle valves is indicated by and checked at additional slip rings. The throttle valves can be also manually adjusted or set.
In that process, air rates which have been ascertained empirically or by calculation are associated with respective positions of the throttle valves. However, particularly where nozzle blocks are used, these air rates may change as a result of deposits formed at the outlet openings or of changes of the pressure in the kiln so that air at rather different rates may be injected for a given position of the throttle valves. In addition, wrong control actions may be caused by errors made in the measurement of temperatures and such errors may be due to deposits upon thermocouples.
It is also known that the gas rate can be measured by a measuring instrument which comprises a float, which is disposed in a frustoconical passage and is raised to a greater or lesser extent depending on the rate at which gas flows through.
A hollow needle is secured to the float and indicates the gas rate in a sight tube. Shortly above the mark corresponding to the desired gas rate, an adjustable stop is provided for limiting the rise of the hollow needle so that the float cannot fall during the rotation of the kiln and permit gas to enter the kiln at much higher rates. This instrument can be used for an exact measurement only when the instrument is at the top of the kiln in a vertical orientation. Besides, the stop which limits the rise of the hollow needle must be adjusted whenever the desired rate is changed (German patent specification No. 12 36 216).